1. Field of the Invention
This disclosure relates to luminescent layers suitable for light-emitting devices, such as laminated translucent and transparent ceramic elements and methods for making the same.
2. Description of the Related Art
Solid state light-emitting devices such as light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs) sometimes called organic electroluminescent devices (OELs), and inorganic electroluminescent devices (IEL) have been widely utilized for various applications such as flat panel displays, indicators for various instruments, signboards, and ornamental illuminations, etc. As the emission efficiency of these light-emitting devices continues to improve, applications that require much higher luminance intensity, such as automobile headlights and general lighting, may soon become feasible. For these applications, white LED is one of the promising candidates and has attracted much attention.
Conventional white LED's are manufactured based on a combination of blue LED and yellow light-emitting YAG:Ce phosphor powder used as a wavelength-converting material dispersed in an encapsulant resin such as epoxy and silicone, as disclosed in U.S. Pat. No. 5,998,925 and U.S. Pat. No. 6,069,440. The wavelength-converting material is so disposed as to absorb some part of the blue LED light-emission and re-emit the light at a different wavelength as yellow or green-yellow light. The combination of the blue light from the LED and the green-yellow light from the phosphor results in perceived white light. However, since the particle size of YAG:Ce phosphor powder utilized for this system is around 1-10 μm, the YAG:Ce powder dispersed in the transparent matrix can cause strong light scattering. As a result, a considerable portion of both incident light from the blue LED and yellow light emitted from the YAG:Ce powder ends up being backscattered and dissipated, causing a loss of white light emission.
One solution to this problem is to form a monolithic ceramic member as a wavelength-converting material. The ceramic member can be constituted by plural ceramic layers of single or multiple phosphors, or transparent layers. The transparent ceramic layers may be constituted by, for example, the same host material as the wavelength-converting material, but may be devoid of any dopant (U.S. Pat. No. 7,361,938). These laminated layers may also be in the form of luminescent ceramic cast tapes, which can be laminated and co-fired (U.S. Pat. No. 7,514,721 and U.S. Published Application No. 2009/0108507). However, since these laminated layers are generally formed from garnet powder of low IQE (Internal Quantum Efficiency) produced through solid state reaction or co-precipitation, the present inventors recognized that the resultant luminosity generated by these luminescent layers is poor even though the cost of manufacture is low. Phosphor nanoparticles produced by radio frequency thermal plasma treatment of liquid precursors showed high wavelength conversion efficiency (WO2008/112710) and very well controlled stoichiometry, but generally have high production costs. As a result, monolithic ceramic plates composed entirely of plasma nanoparticles would increase production costs.
As described in U.S. Patent Application Publication 2004/0145308 A1, U.S. Pat. No. 7,361,938 B2, and U.S. Pat. No. 7,514,721 B2, white light emitting LED's using luminescent ceramic as a wavelength converter is a promising configuration for high power white light emitting LED's. A typical device structure is shown in FIGS. 1A and 1B. A submount 10 shown in FIG. 1A has a blue LED 11 mounted thereon, with a ceramic wavelength converter 12 disposed thereon, encapsulated by a protective resin 15. Plural electrical connections 16, in this embodiment in the form of protrusions extending from either the blue LED 11 or submount 10 electrically communicate the blue LED with a power source (not shown). In FIG. 1A, since typical size of LED chip is less than 1 mm×1 mm, the size of wavelength converter 12 is generally about the same size.
As shown in FIG. 1B, the ceramic wavelength converter 12 is spaced apart from but positioned to receive blue light emitted from the blue LED 11. Protective resin 15 is disposed in the space defined between the ceramic wavelength converter 12 and the blue LED 11. Electrical connections 16, in this embodiment in the form of wires, extend into the defined space and electrically communicate the blue LED with the power source. Die bonding 18 conjoins the blue LED 11 to the submount/package 10. In FIG. 1B, the size of luminescent ceramic 12 can be similarly sized in an LED package, but can be less than 10 mm×10 mm, and often, less than 5 mm×5 mm. However, rigid ceramic plates are generally prepared in sizes much larger than these sizes, thus there is a need for them to be diced and cut into such small pieces from a larger prepared ceramic mother plate. Some have attempted to solve this problem by forming green sheets of the pre-sintered ceramic material, then cutting the green sheet. (U.S. Pat. No. 7,361,938; US Published Application 2009/0108507). Some have described notching or punching the tape or stack and then snap breaking the ceramic material (U.S. Pat. No. 7,514,721). In cutting the green sheets, dicing techniques using a diamond blade have been utilized. In this dicing process, dimension accuracy is very important in order to reproduce a consistent white color from so constructed LED. For example, if the size of the luminescent ceramic is larger, the emission color can be yellowish white, whereas if it is smaller, the color can be bluish. Furthermore, these dicing processes are costly and time consuming. In addition, the width of chips lost using a diamond blade is not a negligible loss because the size of the luminescent ceramic itself is so small.
In addition, in the production process of semiconductor devices, semiconductor chips are manufactured by forming a circuit in a large number of areas sectioned by streets (cutting lines) formed on the front surface of a substantially disk-like semiconductor wafer in a lattice form and by dividing the areas having the circuit formed therein along the streets. As a result, various methods and constructs have been described for dicing semiconductor wafers (U.S. Pat. No. 5,169,804; U.S. Pat. No. 7,129,150; U.S. Pat. No. 7,670,872). However, these wafers are not translucent. When these individual translucent ceramics are separated from a larger form, color fluctuations between individual ceramic elements can be discerned among the plural elements. Further, since these semiconductor chips have circuitry formed therein, and manufacturing parameters of circuitry manufacturers, such as exposure time of chips to heat, are different, sufficient guidance for the manufacture of translucent ceramic materials has not yet been provided.
The present inventors recognized that manufacture of plural luminescent ceramic tiles is fraught with functional variations. Thus, the present inventors recognized that there is a need for an effective way to produce plural luminescent ceramic tiles while reducing color variation amongst the produced tiles.